linux-kernel - Re: [PATCH] perf tools: Provide backward compatibility with previous perf.data version

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Date:	Mon, 12 Oct 2009 11:10:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] perf tools: Provide backward compatibility with
	previous perf.data version


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Thu, Oct 08, 2009 at 10:07:29PM +0200, Frederic Weisbecker wrote:
> > We have merged the trace.info file into perf.data by adding one section
> > in the perf headers. This makes it incompatible with previous version:
> > the new perf tools can't read the older perf.data.
> > 
> > To support the previous format, we check the headers size. If they
> > have the same size than in the previous format, then ignore the trace
> > info section that doesn't exist.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Cc: Mike Galbraith <efault@....de>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > ---
> >  tools/perf/util/header.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index 212fade..9aae360 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -287,10 +287,16 @@ struct perf_header *perf_header__read(int fd)
> >  	do_read(fd, &f_header, sizeof(f_header));
> >  
> >  	if (f_header.magic	!= PERF_MAGIC		||
> > -	    f_header.size	!= sizeof(f_header)	||
> 
> 
> 
> Few notes about this.
> I can send a separate fix for .32 that would just consist in the above line,
> or more likely I can replace it by:
> 
> 	f_header.size < sizeof(f_header)     ||
> 
> and then we'll get a minimal forward compatibility from the older
> tools (can be Cc'ed to stable for .31).

Yep, would be nice.

> Another thing. We may feel the need to add yet another sections in the 
> future.
> 
> So just a suggestion: we could turn this trace_info section into a 
> more generic one in which we could add as much subsections as we want 
> in the future while always ensuring backward compatibility. That could 
> be managed through a versioning of this generic section.

Definitely! (Since it's not upstream yet we can do it without supporting 
the interim version.)

	Ingo
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